Digitizers, as known in the art, are very useful for translating the position of an item in a plan or drawing into coordinates recognizable by a computer. In the latter form, the computer can make use of the position information in any number of useful ways. Unlike the position information from a mouse, which is always relative to its immediately previous position on a support surface, the digitizer position information is relative to a grid of conductors incorporated into its work surface. It becomes, therefore, absolute with respect to the work surface and any plan or drawing mounted thereon in a designated orientation. As a result, the position information is accurate enough not only to edit graphic displays, but also to control manufacturing processes according to scale drawings placed on the work surface, and to control navigation of air and water craft according to charts.
A number of well known digitizer systems make use of a movable coil and a work surface defined by a grid of conductors. The coil may be disposed within the tip of a pen-shaped instrument so that the pen point accurately locates the effective coil center. Alternatively, the coil may surround a transparent disc with a set of cross hairs etched thereon to mark the coil center in what is known as a cursor. The grid normally comprises a set of parallel conductors spaced along the work surface in what may be called the x direction and another set similarly disposed along the work surface in the orthogonal y direction. An oscillator applies an ac signal of predetermined frequency and amplitude to the coil, which is inductively coupled to the conductors of the grid.
In accordance with the principles of well known electromagnetic theory, ac electrical signals are induced in the grid conductors at a magnitude and phase that depend on the location of the coil relative to the conductors. Generally, the signals induced in the conductors will have a magnitude that varies from zero at the coil center to a maximum at the coil periphery and tapering off beyond. Further, the phase of the signals induced in conductors at one side of the coil will be the opposite of (180 degrees displaced from) that of signals induced in conductors at the other side. Both of these properties are used to advantage in several known digitizers.
In the digitizer disclosed in U.S. Pat. No. 4,423,286, that issued to Gary A. Bergeron Dec. 12, 1983, individual conductors are selectively connected, one at a time, through the use of multiplexer circuitry, to detection circuitry that determines both the phase and the magnitude of the signal induced in the selected conductor. When a conductor of the x set is selected by the multiplexer circuitry, the phase of the induced signal indicates whether the pointer is to the right or to the left of the selected conductor. By first selecting a conductor at the center of the work surface, the Bergeron apparatus determines whether the pointer is on the right half or the left half of the surface. A conductor near the center of the indicated half is next selected to determine the quarter of the work surface on which the pointer is located. Successive conductors are similarly selected in a logical progression to very quickly identify the two adjacent conductors in which the induced signals are of opposite phase, indicating that the pointer is located between them. The precise position of the pointer between the identified adjacent conductors is then determined by the relative magnitudes of induced signal in the identified conductors.
In other digitizers, such as that described in U.S. Pat. No. 4,734,546, that issued to Waldo L. Landmeier Mar. 29, 1988, each conductor is looped through several areas of the work surface to reduce the number of conductors in each grid set. Looping, of course, generates segments of each conductor in which the direction across the work surface of a continuous conductor current is reversed. These segment direction reversals are observed by the detecting circuitry as phase reversals of the induced signal. Each conductor is looped in a different manner so that, in each work surface area, the combination of conductor segment directions is unique. Since the phase of the signals induced in all conductor segments to one side of the pointer is the same, the pattern of observed phase reversals among the combination of conductors is used to uniquely identify the particular area of the work surface on which the pointer coil is located. Phase reversals due to segment loop direction are then compensated for, and pointer location within the area proceeds normally.
One disadvantage of these known digitizer systems is the electrical cable that normally carries the ac signal to the coil. Such cables tether the pointer to the base electronics and restrict free movement of the pointer across the digitizer work surface. The oscillator can, of course, be located in the pointer together with a battery for power. A shortcoming of this arrangement until now, however, has been that the oscillator phase reference needed for these improved systems would normally be lost.
An object of the present invention is to provide a digitizer system having an untethered pointer that provides an oscillator phase reference signal to the receiving circuitry.